WO2025017973A1 - Method for dephosphorizing molten iron - Google Patents

Abstract

To provide a method for dephosphorizing molten iron with which it is possible to effectively improve phosphorus distribution and promote dephosphorization reaction of molten iron by imparting electric energy by an electrochemical method. This method for dephosphorizing molten iron, in which an electrode in contact with molten iron is used as a positive electrode and an electrode in contact with only molten slag is used as a negative electrode, and a current is applied between the molten slag and the molten iron through the electrodes, is characterized in that the applied current density I of the current satisfies the following relational expression (1) with respect to the molten steel temperature T of the molten iron, the phosphorus distribution Lp of the molten slag, and the required phosphorus distribution Lp' of the molten slag. [Expression 1]: I ≥ α × In (Lp'/Lp) × T In the relational expression (1), I is the applied current density (A/m²), α is a constant, Lp is the phosphorus distribution (-) of the molten slag, Lp' is the required phosphorus distribution (-) of the molten slag, and T is the molten steel temperature (K) of the molten iron.

Description

Method for dephosphorizing molten iron

The present invention relates to a method for dephosphorization of molten iron. In particular, the present invention relates to a method for dephosphorization of molten iron that can accelerate the dephosphorization reaction.

The phosphorus contained in molten steel tends to segregate at grain boundaries in the material formed from the molten steel, reducing the strength and toughness of the steel formed from the molten steel. For this reason, the phosphorus content in the molten steel is generally controlled to be low when producing steel from molten steel.

In recent years, there has been an increasing trend towards producing high-grade steel, and there is a demand to further reduce the phosphorus content in molten steel. However, due to factors such as the deterioration of the quality of iron ore, the raw material for molten steel, the load imposed by the dephosphorization process on molten iron is becoming increasingly severe. From this technical perspective, it is essential to develop technology that can reduce the phosphorus concentration in molten steel after it has been processed.

In conventional dephosphorization technology for molten steel, since the phosphorus oxide contained in the molten steel is acidic, lime-based flux such as CaO is added to the molten steel to increase the basicity of the molten slag formed during the production of molten steel. By adding lime-based flux such as CaO to the molten steel, the dephosphorization ability of the molten slag can be improved and the equilibrium phosphorus concentration of the molten steel can be reduced.

However, molten slag with high basicity has a high melting temperature, and when the molten steel temperature drops during hot metal pretreatment, the viscosity of the molten slag increases. This leads to insufficient slag formation of CaO contained in the lime-based flux absorbed into the molten steel, which tends to reduce the dephosphorization efficiency. In order to increase this low dephosphorization efficiency and compensate for the reaction efficiency of the dephosphorization reaction, it is necessary to use excess refining material in the molten steel. If excess refining material is used in the molten steel, the cost of refining the molten steel increases. Furthermore, when attempting to dephosphorize the phosphorus concentration in the molten steel to an extremely low phosphorus concentration, the amount of molten slag becomes excessive. As a result, problems such as insufficient capacity of the molten steel refining equipment and slag removal equipment arise.

Therefore, in the dephosphorization process of molten steel aiming at extremely low phosphorus, a method has been adopted in which halides such as fluorite ( _CaF2 ) are added to promote the formation of slag from CaO contained in the lime-based flux and reduce the amount of flux added, thereby achieving both dephosphorization capacity and a reduction in the amount of molten slag, thereby maintaining a high dephosphorization rate up to the extremely low phosphorus region.

However, adding halides such as fluorite (CaF ₂ ) to molten steel increases the content of fluorine (F) and other elements in the molten slag that is formed. In recent years, with increasing social interest in environmental issues, use of fluorine (F) is restricted because of the problem of elution. From this perspective, it has become difficult to use halides such as fluorite (CaF ₂ ) in molten steel. Therefore, there is a demand for a technology that can efficiently dephosphorize phosphorus in molten steel without increasing the basicity of the slag.

In light of the above situation, research has been conducted on technology that focuses on electrical energy and efficiently removes phosphorus from molten steel without increasing the basicity of the molten slag. For example, Non-Patent Document 1 discloses a method for promoting dephosphorization reactions that organizes the reaction that occurs at the interface between slag and molten iron (hereinafter referred to as slag-metal reaction) based on the concept of electrochemistry. According to Non-Patent Document 1, the concept of promoting reactions using electrical energy is presented, using as a concrete example the fact that iron is oxidized and removed in the slag by polarizing the potential of molten iron to the noble side, and conversely, when it is polarized to the noble side, the iron ions in the slag are reduced and returned to the molten iron. Since then, various research related to slag-metal reactions have been reported based on Non-Patent Document 1.

Patent Document 1 discloses a method for reducing the content of granular iron in molten slag and the variation in the metallic iron content in the slag for each charge by passing an electric current through an electrode in contact with the molten slag and an electrode in contact with the iron bath.

Patent Document 2 discloses a method for transferring impurities such as phosphorus and sulfur in steelmaking slag to the molten scrap iron that has sunk to the bottom of the furnace and absorbing them to be recycled as steelmaking slag. The method for recycling steelmaking slag described in Patent Document 2 reduces the phosphorus and sulfur in the slag by applying an electric current to an electrode on the slag side as the anode and an electrode on the molten scrap iron side as the cathode, and transfers them into the molten scrap iron.

Patent No. 7158570 specification Japanese Patent Application Publication No. 11-302719

Tokuda, Masanori, "Coupling Phenomena in Slag-Metal Reactions," Journal of the Japan Institute of Metals, Vol. 15, No. 6 (1976), accepted February 17, 1976

However, such conventional technology has the following problems that need to be solved. That is, Non-Patent Document 1 discloses a method for accelerating the dephosphorization reaction after electrochemically sorting out the slag-metal reaction. However, all of the methods for accelerating the dephosphorization reaction of molten iron described in Non-Patent Document 1 involve changing the mixed potential of the dephosphorization reaction system of molten iron or the equilibrium potential of the phosphorus reaction by using additives or the like, and do not accelerate the dephosphorization reaction by applying electrical energy from the outside. On the other hand, the technology disclosed in Patent Document 1 applies an electric current between the slag and metal. And, in the molten slag treatment method described in Patent Document 1, the purpose of applying an electric current between the slag and metal is to reduce the content of granular iron present in the slag and the variation of the granular iron. In other words, in the molten slag treatment method described in Patent Document 1, the effect of passing an electric current between the slag and metal is to promote the aggregation and coarsening of granular iron, so the effect on the electrochemical reaction is not taken into consideration, and the dephosphorization reaction from molten iron is not accelerated.

In the technology disclosed in Patent Document 2, electrical energy is used for current heating to melt the molten slag and scrap iron, and does not contribute to the electrochemical reaction. In addition, the technology disclosed in Patent Document 2 promotes so-called rephosphorization, which removes phosphorus from molten slag and transfers it to molten iron, but does not promote the dephosphorization reaction from molten iron.

The present invention was made in consideration of the above circumstances, and aims to provide a method for dephosphorizing molten iron that can effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron by applying electrical energy using an electrochemical method.

The inventors conducted various experiments to solve the above problems, and discovered that in a method of dephosphorizing molten iron in which an electric current is applied between molten slag and molten iron, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron by controlling the applied current based on the relationship between phosphorus distribution in the molten slag, the required phosphorus distribution in the molten slag, and the molten steel temperature of the molten iron. The present invention was made based on the above findings, and its gist is as follows.

　関係式（１）において、Ｉは印加電流密度（Ａ／ｍ^２）であり、αは定数であり、ＬＰは溶融スラグの燐分配（－）であり、ＬＰ’は溶融スラグの必要燐分配（－）であり、Ｔは溶鉄の溶鋼温度（Ｋ）である。 That is, the present invention provides a method for dephosphorizing molten iron that advantageously solves the above-mentioned problems, comprising applying a current between the molten slag and the molten iron through an electrode that is in contact with the molten iron as an anode and an electrode that is in contact only with the molten slag as a cathode, wherein the applied current density I of the current satisfies the following relationship (1) between the molten steel temperature T of the molten iron, the phosphorus distribution in the molten slag LP, and the required phosphorus distribution in the molten slag LP':

In the relational equation (1), I is the applied current density (A/m ² ), α is a constant, LP is the phosphorus distribution in the molten slag (-), LP' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature of the molten iron (K).

　関係式（２）において、Ｉは印加電流密度（Ａ／ｍ^２）であり、ＬＰは溶融スラグの燐分配（－）であり、ＬＰ’は溶融スラグの必要燐分配（－）であり、Ｔは前記溶鉄の溶鋼温度（Ｋ）である。
（ｂ）前記溶融スラグと前記溶鉄との間に前記電流を印加することによってアーク放電が発生しないこと、
（ｃ）前記電流の電流値が５０００（Ａ）以下であること、
（ｄ）前記溶融スラグの液相率が６０ｖｏｌ.％以上であること等がより好ましい解決手段になり得るものと考えられる。 The method for dephosphorizing molten iron according to the present invention includes the steps of:
(a) the carbon concentration [C] of carbon contained in the molten iron is 4.0 mass% or less, and the applied current density I (A/m ² ) satisfies the following relational expression (2);

In the relational expression (2), I is the applied current density (A/m ² ), LP is the phosphorus distribution in the molten slag (-), LP' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature of the molten iron (K).
(b) no arc discharge is generated by applying the current between the molten slag and the molten iron;
(c) the current value of the current is 5000 (A) or less;
(d) The liquid phase rate of the molten slag is considered to be 60 vol.% or more, which is considered to be a more preferable solution.

According to the present invention, by applying electrical energy to molten iron using an electrochemical method, it is possible to effectively improve phosphorus distribution without modifying the slag and promote the dephosphorization reaction of molten iron.

1 is a schematic diagram showing an overview of a molten iron dephosphorization apparatus used to carry out a molten iron dephosphorization method according to an embodiment of the present invention. 1 is a graph showing the relationship between the dephosphorization treatment time (min) of molten iron and the phosphorus concentration (mass %) when the dephosphorization method of molten iron according to the present embodiment is carried out. 1 is a graph showing the relationship between the applied current density and phosphorus distribution of phosphorus contained in molten iron when a current is applied to electrodes using a molten iron dephosphorization apparatus.

[First embodiment]
A method for dephosphorizing molten iron according to the first embodiment will be described. The method for dephosphorizing molten iron according to this embodiment is a method for dephosphorizing molten iron in which an electrode in contact with the molten iron is an anode and an electrode in contact only with the molten slag is a cathode, and a current is applied between the molten slag and the molten iron through both electrodes. A molten iron dephosphorization apparatus for implementing the method for dephosphorizing molten iron according to this embodiment will be described.

<Outline of molten iron dephosphorization equipment>
Fig. 1 is a schematic diagram showing an outline of a molten iron dephosphorization apparatus used in carrying out the molten iron dephosphorization method according to the present embodiment. As shown in Fig. 1, the molten iron dephosphorization apparatus 100 includes an MgO crucible 101, a ramming material 102, and an induction melting furnace 103. The MgO crucible 101 is charged with molten iron formed by melting scrap, molten iron, etc. The MgO crucible 101 is preferably made of a material that has low solubility in molten iron and is thermodynamically stable.
In addition to the MgO crucible 101, a CaO crucible or an Al ₂ O ₃ crucible can be used as a crucible. The shape of the MgO crucible 101 is not particularly limited, but may be cylindrical. When the MgO crucible 101 is cylindrical, its cross-sectional area may be 0.005 to 0.030 m ² , for example, 0.018 m ² .

The ramming material 102 is a refractory material. The ramming material 102 covers the side walls and bottom surface of the MgO crucible 101, and is attached to the inner wall of the induction melting furnace 103. From the viewpoint of heat resistance and durability, the thickness of the ramming material 102 is preferably 100 to 150 mm. The induction melting furnace 103 is a crucible-type low-frequency induction furnace. The induction melting furnace 103 can hold molten iron while maintaining a high temperature. An induction heater may be provided on the side wall of the induction melting furnace 103.

Furthermore, the molten iron dephosphorization apparatus 100 is equipped with a cathode 104 and an anode 105 for applying a current between the molten slag 200 and the molten iron 300 charged in the MgO crucible 101. The cathode 104 is in contact only with the molten slag 200, and does not contact the molten iron 300 present on the underside of the molten slag 200. The cathode 104 is preferably formed from a heat-resistant material since the molten slag temperature of the molten slag 200 charged in the MgO crucible 101 becomes extremely high. The material for forming the cathode 104 is preferably graphite or artificial graphite. The cathode 104 may be in the shape of a rod or a plate.

The anode 105 is in contact with the molten iron 300. The anode 105 is in contact with the molten slag 200 and the molten iron 300. The anode 105 is preferably made of a heat-resistant material because the molten steel temperature of the molten slag 200 and the molten iron 300 charged in the MgO crucible 101 becomes extremely high. The material for the anode 105 may be a composite material such as carbon or C-MgO. The form of the anode 105 may be, for example, the core metal part of a stirring lance that is immersed in the molten iron (molten metal) to stir the molten iron (molten metal) by blowing in an inert gas such as argon gas or nitrogen gas, or a graphite-containing refractory brick installed below the surface of the molten iron (molten metal).

The cathode 104 attached to the induction melting furnace 103 is connected to the negative pole of the DC power supply 106 installed outside the induction melting furnace 103 via a conductor 107. The anode 105 attached to the induction melting furnace 103 is connected to the positive pole of the DC power supply 106 installed outside the induction melting furnace 103 via a conductor 107. In this way, an electric circuit is formed by connecting both electrodes consisting of the cathode 104 and the anode 105 to the DC power supply 106 installed outside the induction melting furnace 103.

The top surfaces of the MgO crucible 101, ramming material 102, and induction melting furnace 103 provided in the molten iron dephosphorization apparatus 100 are covered with an insulating board 108. The insulating board 108 covers the top surfaces of the MgO crucible 101, ramming material 102, and induction melting furnace 103, and serves to maintain the temperature of the molten iron 300 charged into the MgO crucible 101. There are no particular limitations on the insulating board 108, so long as it is made of a material that has insulating properties.

<Dephosphorization of molten iron by applying electric current>
In the method for dephosphorizing molten iron according to this embodiment, an electrode in contact with the molten iron is used as an anode, and an electrode in contact only with the molten slag is used as a cathode, and a current is applied between the molten slag and the molten iron through both electrodes.
Hereinafter, the dephosphorization of molten iron by applying an electric current in the method for dephosphorization of molten iron according to this embodiment will be described.

Industrially pure iron is charged into the MgO crucible 101. Ramming material 102 is embedded into the outer wall of the MgO crucible 101, and the industrially pure iron is heated and melted using an induction melting furnace 103 to produce molten iron 300. The phosphorus concentration of the phosphorus contained in the molten iron 300 is adjusted to be within a predetermined range to produce the molten iron 300. Here, the phosphorus concentration of the phosphorus contained in the molten iron 300 may be 0.01 to 0.20 mass%, preferably 0.05 to 0.15 mass%, and more preferably 0.08 mass%. The total amount of the molten iron 300 produced by melting the industrially pure iron can be set to 5 to 30 kg, preferably 10 to 20 kg, and more preferably 15 kg.

Furthermore, within the molten iron dephosphorization apparatus 100 system, flux is added to the upper surface of the molten iron 300 to form molten slag 200 on the upper surface of the molten iron 300. The amount of flux added can be determined appropriately depending on the internal volume of the MgO crucible 101, the total amount of molten iron 300, etc. The amount of flux added may be, for example, 10 to 30 kg/molten iron-t, preferably 15 to 25 kg/molten iron-t, and more preferably 20 kg/molten iron-t.

The compositional components of the flux are not particularly limited as long as the flux contains compositional components capable of forming the molten slag 200. The compositional components of the flux may contain, for example, CaO, SiO ₂ , FeO, MgO, etc. When the flux contains CaO, SiO ₂ , FeO, and MgO as its compositional components, the content thereof may be 22.5 (% CaO), 28.0 (% SiO ₂ ), 42.5 (% FeO), and 7.0 (% MgO) on a mass basis.

After the flux is poured onto the top surface of the molten iron 300, the molten steel temperature of the molten iron 300 present inside the MgO crucible 101 is maintained in the range of 1300 to 1700°C, preferably 1585 to 1615°C. By pouring the flux into the molten iron 300 and maintaining that temperature, molten slag 200 and molten iron 300 are formed within the system of the molten iron dephosphorization apparatus 100. The molten slag 200 is formed on the surface of the molten iron 300. A molten slag 200 (slag)-molten iron 300 (metal) interface is formed between the molten slag 200 and the molten iron 300.
The molten slag 200 thus formed has a molten slag composition that allows the insertion of a cathode 104 and an anode 105 into the molten slag 200 for use in applying an electric current between the molten slag 200 and the molten iron.

The cathode 104 is immersed in the molten slag 200 formed in the system of the molten iron dephosphorization apparatus 100. The cathode 104 is immersed only in the molten slag 200. Meanwhile, the anode 105 is immersed in the molten slag 200 and molten iron 300 formed in the system of the molten iron dephosphorization apparatus 100. That is, the anode 105 may be immersed in the molten slag 200 and molten iron 300 formed from C-MgO bricks, which are carbon-containing refractories. In this way, a DC power source 106 is used between the two electrodes consisting of the cathode 104 and anode 105 installed in the molten iron dephosphorization apparatus 100, and a DC current is applied to the molten slag 200 and molten iron 300.

　関係式（１）において、Ｉは印加電流密度（Ａ／ｍ^２）であり、αは定数であり、ＬＰは溶融スラグの燐分配（－）であり、ＬＰ’は溶融スラグの必要燐分配（－）であり、Ｔは溶鉄の溶鋼温度（Ｋ）である。 The current density applied between the electrodes is preferably determined taking into consideration the dephosphorization treatment time, the required phosphorus distribution considering the target phosphorus concentration, and the power cost. Specifically, by setting the current density applied between the electrodes in order to increase the current density applied between the electrodes, the attainable phosphorus concentration can be reduced and the dephosphorization rate can be increased.
For this reason, by applying a large current between the two electrodes, the processing time required for dephosphorization of molten iron can be shortened and molten iron (molten metal) with a phosphorus concentration equal to or lower than the target concentration can be obtained, but the power cost is excessively high. From such a technical viewpoint, a current can be applied between the electrodes by setting the applied current density so as to satisfy the following relational expression (1).

In the relational equation (1), I is the applied current density (A/m ² ), α is a constant, LP is the phosphorus distribution in the molten slag (-), LP' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature of the molten iron (K).

In the method for dephosphorizing molten iron according to this embodiment, the value of the applied current density I calculated using relational expression (1) is the current density that can obtain the minimum effect of dephosphorizing molten iron. Here, there are two main effects when a current density equal to or greater than the value of the applied current density I calculated using relational expression (1) is applied between the molten slag 200 and the molten iron 300.

First, as can be seen from the later-described relational expression (5), the productivity of molten steel can be improved by increasing the reaction rate of the dephosphorization reaction of molten iron.
Secondly, the cost of electricity required to dephosphorize molten iron increases. In other words, if the applied current density used to dephosphorize molten iron is increased more than necessary, both the productivity and production costs increase. Therefore, it is desirable to determine the upper limit of the applied current density used to dephosphorize molten iron, taking into account the allowable time and cost of operation to dephosphorize molten iron, the allowable current of the power source, etc.
From such a technical viewpoint, it is extremely significant to be able to calculate the value of the applied current density I using the relational expression (1) since it can ensure the dephosphorization effect of molten iron and provide suitable conditions for the method of dephosphorization of molten iron taking into consideration the allowable operation time, cost, and allowable current of the power source for carrying out the dephosphorization of molten iron.

Specifically, in the method for dephosphorizing molten iron according to this embodiment, the value of the applied current density I that can be set using the relational expression (1) is 150 to 600 (A/m ² ), and preferably 200 to 500 (A/m ² ). If the value of the applied current density I is 150 (A/m ² ) or more, it is preferable because it is possible to reduce the phosphorus concentration of the phosphorus contained in the molten iron 300 that can be achieved and increase the dephosphorization rate. If the value of the applied current density I is 600 (A/m ² ) or less, it is preferable because it is possible to suppress the power cost when dephosphorizing the molten iron 300.
In the dephosphorization method of molten iron according to this embodiment, the phosphorus concentration of phosphorus contained in the molten iron 300 can be measured after a certain time has elapsed since the start of application of a current between the two electrodes.

Figure 2 is a graph showing the change in phosphorus concentration of phosphorus contained in molten iron over time when dephosphorization of molten iron is performed by applying a current between the electrodes using a molten iron dephosphorization device. In other words, Figure 2 shows the relationship between the dephosphorization process time (min) and the phosphorus concentration (mass%) of molten iron. As shown in Figure 2, the larger the applied current value, the higher the dephosphorization speed and the lower the achieved phosphorus concentration.

Figure 3 is a graph showing the relationship between the applied current density and the phosphorus distribution of phosphorus contained in the molten slag when molten iron is dephosphorized by applying a current to the electrodes using a molten iron dephosphorization apparatus. As shown in Figure 3, it can be seen that the logarithm of the phosphorus distribution Lp contained in the molten slag 200 tends to increase linearly with the applied current density I applied to both electrodes. This tendency remains the same even if the component composition of the molten slag 200 is changed. Furthermore, this tendency is maintained even when the molten iron 300 is dephosphorized in combination with stirring the molten iron 300 with a bubbling lance. Furthermore, the inventors have confirmed that the logarithm of the phosphorus distribution of the molten slag 200 increases linearly with the applied current density I, even when the molten steel temperature T is changed when the dephosphorization method for the molten iron 300 is applied.

The principle by which the molten iron dephosphorization method according to this embodiment can promote the dephosphorization reaction of molten iron is believed to be as follows. That is, by applying a current to the molten iron 300 and the molten slag 200, the potential on the molten iron 300 side becomes more noble and the potential on the molten slag 200 side becomes more base through both electrodes. The potential change at this time is called overvoltage. Here, due to the change in Gibbs energy equivalent to the overvoltage, the equilibrium reaction equation for the dephosphorization reaction of phosphorus contained in the molten iron 300 and the equilibrium constant K for the dephosphorization reaction of phosphorus are as follows:

By applying a current to both electrodes using the molten iron dephosphorization apparatus 100 to proceed with the dephosphorization reaction of phosphorus contained in the molten iron 300, the phosphorus (P) contained in the molten iron 300 becomes phosphorus ions (P ⁵⁺ ), and the concentration [P ⁵⁺ ] of the phosphorus ions (P ⁵⁺ ) increases. Therefore, the equilibrium constant K of the dephosphorization reaction of phosphorus contained in the molten iron 300 increases. As a result, it is considered that the concentration [P] of phosphorus (P) contained in the molten iron 300 decreases. In this case, if the reaction order of the dephosphorization reaction of phosphorus contained in the molten iron 300 is set to first order and the reaction rate v of the dephosphorization reaction is expressed as a linear equation of the phosphorus concentration [P], the following relational expression (5) is obtained. In relational expression (5), t is the dephosphorization treatment time t(s) of phosphorus contained in the molten iron, k is the apparent reaction rate constant of the dephosphorization reaction of phosphorus contained in the molten iron, and [P]e is the equilibrium phosphorus concentration when the dephosphorization reaction of phosphorus contained in the molten iron reaches dissolution equilibrium.

According to the relational expression (5), the equilibrium phosphorus concentration [P]e decreases when the dephosphorization reaction of phosphorus contained in molten iron 300 reaches dissolution equilibrium, and the reaction rate v of the dephosphorization reaction of phosphorus contained in molten iron 300 increases. In this case, if the molten steel temperature when the dephosphorization reaction of phosphorus contained in molten iron 300 reaches dissolution equilibrium is T (K), the phosphorus distribution of phosphorus contained in molten slag 200 when no current is applied to molten iron 300 and molten slag 200 is Lp, and the required phosphorus distribution of phosphorus contained in molten slag 200 after current is applied to molten iron 300 and molten slag 200 is Lp', the current application density I of the current required to obtain the required phosphorus distribution Lp' is expressed by the following relational expression (1).

　関係式（１）において、Ｉは印加電流密度（Ａ／ｍ^２）であり、αは定数であり、Ｌｐは溶融スラグの燐分配（－）であり、Ｌｐ’は溶融スラグの必要燐分配（－）であり、Ｔは溶鉄の溶鋼温度（Ｋ）である。

In the relational equation (1), I is the applied current density (A/m ² ), α is a constant, Lp is the phosphorus distribution in the molten slag (-), Lp' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature of the molten iron (K).

That is, in the relational expression (1), the phosphorus concentration C _slag of phosphorus contained in the molten slag and the phosphorus concentration C _iron of phosphorus contained in the molten iron at the molten steel temperature T (K) after a predetermined time has elapsed are measured to calculate the phosphorus distribution Lp(-) of the molten slag 200. Next, a current is applied to both electrodes using the molten iron dephosphorization apparatus 100 to determine the required phosphorus distribution Lp'(-) of the molten slag 200 when the dephosphorization reaction of the phosphorus contained in the molten iron 300 progresses and reaches dissolution equilibrium.
Then, the phosphorus distribution Lp(-) of the molten slag 200 at the molten steel temperature T (K) is calculated, the required phosphorus distribution Lp'(-) of the molten slag 200 is set, and the applied current density I (A/m ² ) corresponding to the required phosphorus distribution Lp'(-) of the molten slag 200 can be calculated using the relational equation (1).

Here, the constant α in the relational expression (1) is calculated as follows. First, the relationship between the applied current density I (A/m ² ) and the overvoltage η generated between the electrodes formed by the cathode 104 and the anode 105 when a current is applied to the molten iron 300 and the molten slag 200 is obtained. The applied current density I (A/m ² ) and the overvoltage η are in a proportional relationship and can be expressed by the following relational expression (6). Therefore, the slope α can be calculated from a graph showing the relationship between the applied current density I (A/m ² ) and the overvoltage η.

On the other hand, when a current is applied to the molten iron 300 and the molten slag 200, the change in Gibbs energy ΔG _initial when the current is applied and the change in Gibbs energy ΔG _equilibrium when the phosphorus in the molten iron reaches the dissolution equilibrium after the current is applied are expressed by the following relational expressions (7) and (8), respectively.

　関係式（７）において、Ｒは気体定数、Ｔは溶鋼温度（Ｋ）、溶融スラグの燐分配Ｌｐ（－）、ａ_slagは溶融スラグ中の燐の活量、a_ironは、溶鉄中の燐の活量を表す。

In the relational equation (7), R is the gas constant, T is the molten steel temperature (K), the phosphorus distribution in the molten slag Lp(-), a _slag is the activity of phosphorus in the molten slag, and a _iron is the activity of phosphorus in the molten iron.

Furthermore, based on the relational expressions (7) to (8), the overvoltage η generated between the electrodes formed by the cathode 104 and the anode 105 when a current is applied to the molten iron 300 and the molten slag 200 is expressed by the following relational expression (9).The constant α in the relational expression (1) can be calculated by comparing the relational expressions (6) and (9).
In the relational expression (9), η represents the overpotential, R represents the gas constant, F represents the Faraday constant, Z represents the valence, Lp represents the phosphorus distribution in the molten slag at the molten steel temperature T (K) (-), and Lp' represents the required phosphorus distribution (-).

In this way, the molten iron dephosphorization method according to this embodiment can determine the current application density I required to obtain the required phosphorus distribution Lp' by setting the phosphorus distribution Lp(-) in the molten slag at the molten steel temperature T (K) and the required phosphorus distribution Lp'(-) in the molten slag and using the relational equation (1) in which the constant α is determined.

As described above, according to the invention of the first embodiment, by controlling the applied current density of the applied current based on the relationship between phosphorus distribution in the molten slag, the required phosphorus distribution in the molten slag, and the molten steel temperature of the molten iron, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of the molten iron.

[Second embodiment]
A method for dephosphorizing molten iron according to a second embodiment will now be described. The method for dephosphorizing molten iron according to this embodiment is characterized in that the carbon concentration [C] of carbon contained in the molten iron is 4.0 mass% or less, and the applied current density I (A/m ² ) satisfies the following relational expression (2):

　関係式（２）において、Ｉは印加電流密度（Ａ／ｍ^２）であり、Ｌｐは溶融スラグの燐分配（－）であり、Ｌｐ’は溶融スラグの必要燐分配（－）であり、Ｔは溶鉄の溶鋼温度（Ｋ）である。以下、本実施形態に係る溶鉄の脱燐方法に含まれる技術的特徴部分について説明する。

In the relational expression (2), I is the applied current density (A/m ² ), Lp is the phosphorus distribution in the molten slag (-), Lp' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature of the molten iron (K). The technical features included in the method for dephosphorizing molten iron according to this embodiment will be described below.

The method for dephosphorizing molten iron according to the present embodiment employs relational expression (2) in which the constant α in relational expression (1) employed in the method for dephosphorizing molten iron according to the above embodiment is calculated and set to α = 5.264 × 10 ^-2 . In other words, the method for dephosphorizing molten steel according to the present embodiment employs relational expression (2) and can obtain the molten slag distribution Lp' by measuring the phosphorus concentration after applying a current having an applied current density I to the molten slag 200 and the molten iron 300.

Furthermore, in the method for dephosphorizing molten iron according to the present embodiment, the current applied to the molten slag and the molten iron is controlled by the current value, so that the method for dephosphorizing molten iron according to the present embodiment can reduce the effect on the above-mentioned change in overvoltage even if the electrical properties of the molten iron change.
Furthermore, by applying a current having an applied current density that satisfies the relational expression (2) employed in the method for dephosphorizing molten steel according to this embodiment to the molten slag and molten iron, it is possible to effectively improve phosphorus distribution without modifying the molten slag, and the method for dephosphorizing molten iron according to this embodiment can be applied to molten iron with various compositional component systems.

In the dephosphorization method of molten iron according to the present embodiment, the carbon concentration [C] of carbon contained in the molten iron to which a current is applied is preferably 4.0 mass% or less. If the carbon concentration [C] of carbon contained in the molten iron to which a current is applied is 4.0 mass% or less, it is preferable because the carbon content that can be contained in the molten iron can be secured. Note that the carbon concentration [C] of carbon contained in the molten iron to which a current is applied may be 0.1 mass% or more.
As described above, the method for dephosphorization of molten iron according to this embodiment is advantageous in that it can be applied without any particular restriction on the carbon concentration within the range of carbon concentrations that can be contained in molten iron.

As described above, according to the invention of the second embodiment, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron without being affected by changes in overvoltage due to changes in the electrical properties of molten iron, or by the concentrations of compositional components contained in the molten iron, such as the carbon concentration [C] and phosphorus concentration [P].

[Third embodiment]
A method for dephosphorizing molten iron according to a third embodiment will now be described. The method for dephosphorizing molten iron according to this embodiment is characterized in that, in the method for dephosphorizing molten iron according to the above-mentioned embodiments, no arc discharge occurs when the current is applied between the molten slag and the molten iron.
Hereinafter, technical features included in the method for dephosphorizing molten iron according to this embodiment will be described.

The method for dephosphorizing molten iron according to this embodiment performs dephosphorization of molten iron under conditions that prevent arc discharge from occurring due to the current applied between the molten slag and the molten iron. In other words, the method for dephosphorizing molten iron according to this embodiment prevents arc discharge from occurring due to the current applied between the molten slag and the molten iron, thereby promoting the reaction between the phosphorus contained in the molten iron and iron oxide, and effectively performing dephosphorization of the molten iron.

The occurrence of arc discharge within the system of the molten iron dephosphorization apparatus due to the current applied between the molten slag and molten iron is undesirable from the viewpoint of promoting the dephosphorization reaction of the molten iron. Below, we will explain the reasons for adopting conditions in the molten iron dephosphorization method according to this embodiment that prevent arc discharge from occurring due to the current applied between the molten slag and molten iron.

In the method for dephosphorizing molten iron according to this embodiment, as shown in the following equilibrium reaction formula (10), phosphorus (P) contained in the molten iron before dephosphorization reacts with iron oxide (FeO) contained in the molten iron to become diphosphorus pentoxide, and iron oxide (FeO) contained in the molten iron before dephosphorization is reduced to iron (Fe). The equilibrium constant Kp in the chemical equilibrium reaction formula (10) is as shown in the following relational formula (11).

　ここで、平衡定数Ｋｐを示す関係式（１１）において、ａ_ｐ２Ｏ５は溶鉄に含まれる五酸化二燐（Ｐ_２Ｏ_５）の活量、ａ_Ｆｅは溶鉄に含まれる鉄（Ｆｅ）の活量、ａ_ｐは溶鉄に含まれる燐（Ｐ）の活量、ａ_ＦｅＯは溶鉄に含まれる酸化鉄（ＦｅＯ）の活量をそれぞれ表す。
　なお、溶鉄に含まれる燐（Ｐ）の活量ａ_ｐは、溶鉄に含まれる燐の燐濃度［Ｐ］と、その活量係数ｆｐを用いて、以下の関係式（１２）により表すことができる。

Here, in relational equation (11) showing the equilibrium constant Kp, a _p2O5 represents the activity of diphosphorus pentoxide (P ₂ O ₅ ) contained in the molten iron, a _Fe represents the activity of iron (Fe) contained in the molten iron, a _p represents the activity of phosphorus (P) contained in the molten iron, and a _FeO represents the activity of iron oxide (FeO) contained in the molten iron.
The activity a _p of phosphorus (P) contained in molten iron can be expressed by the following relational expression (12) using the phosphorus concentration [P] of phosphorus contained in molten iron and its activity coefficient fp.

Furthermore, taking the logarithm of both sides of equation (11) showing the equilibrium constant Kp and rearranging the relationship between the activity of diphosphorus pentoxide (P ₂ O ₅ ), the activity of iron (Fe), the activity of phosphorus (P), and the activity of iron oxide (FeO) contained in the molten iron, gives the following equation (13).

As a result, as is clear from the relational equation (13), it can be understood that the dephosphorization reaction of molten iron shown by the chemical equilibrium reaction equation (10) can be promoted by lowering the reaction temperature T. Generally, in a method for dephosphorization of molten iron, an arc discharge is generated by applying a current between the molten slag and the molten iron, thereby increasing the reaction temperature T of the dephosphorization reaction of molten iron.
From this technical viewpoint, the method for dephosphorization of molten iron according to the present embodiment can promote the dephosphorization reaction of molten iron by preventing the generation of an arc discharge due to the current applied between the molten slag and the molten iron and by lowering the reaction temperature T.

Furthermore, from the chemical equilibrium reaction formula (10) and the relational expression (11) showing the equilibrium constant Kp, it is clear that increasing the activity of iron oxide (FeO) contained in molten iron promotes the reaction between the iron oxide and phosphorus contained in the molten slag, and promotes the dephosphorization reaction of molten iron. In addition, decreasing the activity of diphosphorus pentoxide (P ₂ O ₅ ) can suppress the reverse reaction of the dephosphorization reaction of molten iron caused by the progress of the decomposition reaction of diphosphorus pentoxide (P ₂ O ₅ ) contained in the molten slag.

On the other hand, according to the relational expression (12) showing the activity a _p of phosphorus (P) contained in molten iron, the activity a _p of phosphorus (P) is the product of the phosphorus concentration [P] of phosphorus contained in molten iron and its activity coefficient fp. Here, the activity coefficient fp is larger as the carbon concentration [%C] in the molten steel is higher. Therefore, by increasing the carbon concentration [%C] in the molten steel and increasing the activity coefficient fp, the activity a _p of phosphorus (P) contained in molten iron can be increased.

In the method for dephosphorizing molten iron according to the present embodiment, the current value of the current required to prevent arc discharge from occurring when applying a current between the molten slag and the molten iron is preferably 5000 (A) or less.
In the method for dephosphorizing molten iron according to this embodiment, if the current value applied between the molten slag and the molten iron is 500 to 5000 (A) or less, an arc discharge is not generated, and an energy balance in the dephosphorization reaction of the molten iron can be achieved, thereby ensuring the thermal efficiency of the dephosphorization treatment of the molten iron. From this technical viewpoint, the molten iron dephosphorization apparatus to which the method for dephosphorizing molten iron according to this embodiment can be applied is preferably a DC electric furnace.

DC electric furnaces require less electricity, electrodes and refractories per unit of production, and produce less noise and flicker. Furthermore, by equipping DC electric furnaces with scrap preheating and continuous charging equipment, high-temperature exhaust gas can be used for preheating, and heat loss caused by opening the furnace lid when charging scrap can be prevented, reducing energy consumption.

In addition, DC electric furnaces are increasingly being equipped with scrap preheating and continuous charging equipment, and the eccentric bottom tapping method is being adopted. If the eccentric bottom tapping method is adopted for DC electric furnaces, it is more efficient because steel can be tapped quickly without tilting the furnace body, and it is also preferable in terms of maintaining the cleanliness of the molten steel, as slag is less likely to flow into the ladle during tapping.

As described above, according to the invention of the third embodiment, by preventing arc discharge and lowering the reaction temperature T, and by setting the current value of the current applied between the molten slag and the molten iron to 5000 (A) or less, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of the molten iron. The method for dephosphorization of molten iron according to the third embodiment can be carried out using a DC electric furnace.

[Fourth embodiment]
A method for dephosphorizing molten iron according to a fourth embodiment will now be described. The method for dephosphorizing molten iron according to this embodiment is characterized in that the liquid phase ratio of the molten slag is 60 vol.% or more in the method for dephosphorizing molten iron according to the above-mentioned embodiments. The technical features of the method for dephosphorizing molten iron according to this embodiment will now be described.

In the molten iron dephosphorization method according to this embodiment, molten slag 200 is formed in a molten iron dephosphorization apparatus 100, such as a refining reaction vessel, into which molten iron 300 (molten metal) is charged. At this time, the molten slag 200 is added to the upper surface of the molten iron 300 so that the thickness of the molten slag 200 is such that the cathode 104 can be immersed only in the molten slag 200. An electrode made of a conductive material is immersed only in the molten slag 200 to form the cathode 104.

In the method for dephosphorizing molten iron according to the present embodiment, the components of the molten slag 200 used in the dephosphorization treatment of phosphorus contained in the molten iron 300 are preferably molten slag 200 containing CaO, SiO ₂ , FeO, MgO, etc., which are generally used in dephosphorization refining. According to the method for dephosphorizing molten iron according to the present embodiment, as is clear from the relational expressions (1) and (2), the component composition of the molten slag 200 is not particularly limited, and the required phosphorus distribution of the molten slag 200 can be increased.

In the molten iron dephosphorization method according to this embodiment, the cathode 104 of the molten iron dephosphorization apparatus 100 must be immersed only in the molten slag 200, and the liquid phase ratio of the molten slag 200 is preferably 60 vol.% or more in order to improve the reaction efficiency of the dephosphorization reaction. The liquid phase ratio of the molten slag 200 may be a value that allows the cathode 104 and the anode 105 used to apply a current between the molten slag 200 and the molten iron to be inserted into the molten slag 200. If the liquid phase ratio of the molten slag 200 is 60 vol.% or more, the cathode 104 can be sufficiently immersed in the molten slag 200, and the dephosphorization reaction of the molten iron can be promoted. If the liquid phase ratio of the molten slag 200 is 95 vol.% or less, the operation of the molten iron dephosphorization apparatus 100 is easy, and this is preferable. The liquid phase ratio of the molten slag 200 refers to the ratio of the liquid phase to the molten slag 200.

As described above, according to the invention of the fourth embodiment, by setting the liquid phase ratio of the molten slag to 60 vol.% or more, the cathode of the molten iron dephosphorization device can be sufficiently immersed in the molten slag, effectively improving phosphorus distribution and promoting the dephosphorization reaction of molten iron.

[Other embodiments]
Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the technical scope of the present invention. In addition, programs, systems, or devices that combine the separate features included in each embodiment in any way are also included in the technical scope of the present invention.

The effects of the present invention will be specifically explained below based on examples, but the present invention is not limited to these examples.

(Example 1)
The molten iron dephosphorization method according to the present embodiment was adopted to carry out dephosphorization of molten iron using an electric furnace. Scrap, iron phosphide (FeP), carbonaceous material, and CaO-SiO ₂ -FeO-MgO-based slag were charged into the electric furnace of the electric furnace, and these molten steel raw materials were melted by an AC arc. 300 t of molten steel and 30 kg/t of molten slag were obtained inside the electric furnace.

Next, the graphite electrode on the furnace used for the arc AC was immersed in the molten slag to serve as the cathode. The core metal part of the stirring lance was immersed in the molten steel to serve as the anode. Furthermore, while blowing argon gas (Ar) at 2.0 Nm ³ /min using the stirring lance, a direct current with an average current density of 300 (A/m ² ) and an applied current value of 2100 (A) was applied between the molten slag and the molten steel for 30 minutes to perform dephosphorization of the molten steel (Level 1).
In the dephosphorization of molten steel, samples were taken before the dephosphorization (0 min), 10 min after the start of the dephosphorization, 20 min after the start of the dephosphorization, and 30 min after the start of the dephosphorization (completion of the dephosphorization), and the phosphorus concentration in the molten steel was measured to determine the phosphorus distribution (actual phosphorus distribution). Table 1 shows the phosphorus distribution and slag composition ratio over the dephosphorization time of the molten steel. It also shows the slag liquid phase rate and whether or not the relationship shown in the above relational formula (2) is satisfied. In Example 1, the required phosphorus distribution of the molten slag was set to 100.
Additionally, in the method for dephosphorizing molten iron of Example 1, the occurrence of an arc was confirmed, and the current value of the current applied between the molten slag and the molten steel was set to a predetermined current value.

(Examples 2 to 6)
In Examples 2 to 5, dephosphorization of molten steel was carried out in the same manner as in Example 1, except that the average current density of the direct current applied between the molten slag and the molten steel was changed to 300 to 350 (A/m ² ) and the applied current value was changed to 5000 (A) or less (Levels 2 to 5).
On the other hand, in Example 6, the average current density of the direct current applied between the molten slag and the molten steel was set to 3000 (A/m ² ), and the applied current value was set to 21000 (A) to generate an arc discharge (Level 9).
Table 1 shows the phosphorus distribution (actual phosphorus distribution) and slag composition ratio over the dephosphorization time of molten steel. It also shows the slag liquid phase rate and whether or not the relationship shown in the above-mentioned relational expression (2) is satisfied. In addition, in the dephosphorization methods of molten iron in Examples 2 to 6, the presence or absence of arc generation was confirmed, and the current value applied between the molten slag and molten steel was set to a predetermined current value.

(Comparative Examples 1 to 2)
The dephosphorization treatment of molten steel was carried out in the same manner as in Example 1, without applying a direct current between the molten slag and the molten steel (Level 6). In addition, the average current density of the direct current applied between the molten slag and the molten steel was set to 200 (A/m ² ), and the dephosphorization treatment of molten steel was carried out under conditions that did not satisfy the above-mentioned relational expression (2) (Level 7). Table 1 shows the phosphorus distribution (actual phosphorus distribution) and the slag composition ratio over the detreatment time of the molten steel. Also shown are the slag liquid phase ratio and whether or not the relationship shown in the above-mentioned relational expression (2) was satisfied.

(Comparative Example 3)
The dephosphorization of molten steel was carried out in the same manner as in Example 1, without applying a direct current between the molten slag and the molten steel (Level 8). Table 1 shows the phosphorus distribution (actual phosphorus distribution) and the slag composition ratio over the dephosphorization time of the molten steel. It also shows the slag liquid phase ratio and whether or not the relationship shown in the above-mentioned relational formula (2) is satisfied.

Table 1 shows the test conditions and results at each level adopted in Examples 1 to 6 and Comparative Examples 1 to 3. Table 1 also shows the required phosphorus distribution, and whether the calculated actual phosphorus distribution reaches this is evaluated. As is clear from Table 1, under the condition where the slag liquid phase rate is 100% (Examples 1 to 2, 4 to 6), it was found that under the condition that does not satisfy the above-mentioned relational formula (2), the calculated actual phosphorus distribution (phosphorus distribution obtained from the experimental results) does not reach the required phosphorus distribution (Comparative Example 2).
On the other hand, under the condition that the slag liquid phase ratio is 100% (Invention Examples 1 to 2, 4 to 6), it can be seen that the required phosphorus distribution is achieved under the condition that a direct current having a current density satisfying the above-mentioned relational expression (2) is applied (Invention Examples 1 to 6).
Furthermore, it was found that the required phosphorus distribution can be achieved extremely well even under conditions in which the applied current value is set to 5000 (A) or less and arc discharge is not generated, as shown in Examples 1 to 5. From such a technical viewpoint, it has become clear that the method for dephosphorizing molten iron according to this embodiment can be suitably carried out by a DC electric furnace that can prevent arc discharge from occurring.

Furthermore, it can be seen that the larger the average current density of the DC current applied to the molten steel, the sooner the required phosphorus distribution is reached, and the dephosphorization treatment time can be shortened. Also, when the slag liquid phase ratio is 30%, the solidified slag becomes an obstacle and the electrode cannot be immersed, so the dephosphorization method of molten iron according to the present invention cannot be applied (Comparative Example 3).
However, when the liquid phase rate of the slag is 60 vol.% or more, it is possible to immerse the electrode in the molten slag, but from the viewpoint of the reaction efficiency of the dephosphorization reaction, it is preferable that the liquid phase rate of the slag is 60 vol.% or more. Note that these trends do not depend on the carbon concentration [C] of carbon or the phosphorus concentration [P] of phosphorus contained in the molten iron.

In this way, the method for dephosphorizing molten iron according to the present invention can achieve the required phosphorus distribution when the slag liquid phase ratio is set to 60 vol.% or more and a direct current with a current density that satisfies the above relational expression (2) is applied. In other words, it has become clear that by using the method for dephosphorizing molten iron according to the present invention and setting specific conditions, it is possible to effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron.

The method for dephosphorization of molten iron according to the present invention can effectively improve phosphorus distribution and promote the dephosphorization reaction of molten iron without modifying the slag, which contributes to the development of related industries such as the steelmaking industry and is extremely useful industrially.

100 Molten iron dephosphorization apparatus 101 MgO crucible 102 Ramming material 103 Induction melting furnace 104 Cathode (graphite electrode)
105 Anode (MgO-C electrode)
106 DC power supply 107 Conductor 108 Insulation board 200 Molten slag 300 Molten iron

Claims (5)

A method for dephosphorizing molten iron, comprising the steps of: applying an electric current between the molten slag and the molten iron through an electrode that is in contact with the molten iron as an anode and an electrode that is in contact only with the molten slag as a cathode, the method comprising the steps of:
a current density I of the current satisfying the following relationship (1) between a molten steel temperature T of the molten iron, a phosphorus distribution LP of the molten slag, and a required phosphorus distribution LP' of the molten slag:

In the relational equation (1), I is the applied current density (A/m ² ), α is a constant, LP is the phosphorus distribution in the molten slag (-), LP' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature T (K). The carbon concentration [C] of carbon contained in the molten iron is 4.0 mass% or less,
2. The method for dephosphorizing molten iron according to claim 1, wherein the applied current density I (A/m ² ) satisfies the following relational expression (2):

In the relational expression (2), I is the applied current density (A/m ² ), LP is the phosphorus distribution in the molten slag (-), LP' is the required phosphorus distribution in the molten slag (-), and T is the molten steel temperature of the molten iron (K). The method for dephosphorizing molten iron according to claim 1 or 2, characterized in that no arc discharge occurs by applying the current between the molten slag and the molten iron. The method for dephosphorizing molten iron described in claim 3, characterized in that the current value is 5000 (A) or less. 3. The method for dephosphorizing molten iron according to claim 1, wherein the liquid phase rate of the molten slag is 60 vol. % or more.

Images